Skew time, also referred to hereinafter as “skew”, is a term typically used in electronics and physics to describe a maximum difference between a time in which a signal simultaneously sent from a same source to two or more different destinations first arrives at a destination and last arrives at a destination. Electronic circuits are typically affected by two types of skew times, clock skew and non-clock skew. Clock skew occurs in synchronous circuits and is usually associated with a clock signal from a common clock source arriving at different components at different times. Non-clock skew occurs in synchronous and/or asynchronous circuits and is generally associated with a common signal from a common signal source, other than the clock signal in synchronous circuits, propagating through a circuit and arriving at different components at different times.
A cause of skew is generally attributed to the use of different wire-interconnect lengths between the signal source and different destination components. Several methods are known in the art which attempt to deal with this problem. An example of such method is described in U.S. Pat. No. 5,784,600; “Method of Generating Exact-Length Wires for Routing Critical Signal”, which is incorporated herein by reference. The method describes “an automated method for adjusting wire lengths between connected circuit elements of an integrated circuit which includes the following steps: (1) receiving a value specifying a wire length that must be provided between terminals of two integrated circuits in the integrated circuit design; (2) defining a routing region in which the wire can be routed; and (3) automatically specifying a wire route including a serpentine section with the routing region for connecting the terminals. The serpentine section will include one or more legs sized to ensure that the wire route has the specified wire length. Specifically disclosed is the application of the method to size wiring between two clock buffers in separate and adjacent stages of a clock distribution network.”
Other causes of skew are typically attributed to interconnect capacitive coupling, use of buffers having unequal delays, and/or use of different number of clock buffers between the clock source and clock loads. An example of a method which attempts to remedy these causes is described in U.S. Pat. No. 6,256,766; “Method and apparatus for reducing clock skew”, incorporated herein by reference. Described therein is “a method for reducing skew in a common signal as applied to individual elements in the design phase. In accordance with the principles of the invention, the design of the wiring is established and augmented with compensation elements and/or delay elements as necessary to equalize the skew between relevant components. In the disclosed embodiment, the method generally comprises steps: (1) grouping loads on the common signal; (2) creating a signal wiring tree and inserting delay cells; and (3) providing necessary loading compensation. The loads are grouped such that each utilized node on a central wiring experiences substantially equal loading, with compensating loads added as necessary. The nodes are established at intervals corresponding to the availability of delay elements, which are added to the branches feeding the farthest elements as necessary to equate the time delay of each node with respect to the source of the common signal.”
3D imaging systems generally comprise electro-optical systems adapted to capture and generate three dimensional (3D) images of an object or scene. Their application may be found in many aspects of daily life such as medical imaging systems, TV and movie studio production systems, face recognition systems, surveying systems, robotic guidance systems, and automotive guidance and control systems. CCD (charge coupled device) and CMOS (complementary metal oxide semiconductor) technologies are typically used in image sensors comprised in the 3D imaging systems.
CCD, compared with CMOS, offers advantages of extremely low noise, low dark currents (from thermally generated charge carriers), a high ratio of collected electrons to incident photons, and a high ratio of light sensitive area to pixel size. CMOS technology allows for many circuit functions to be placed on a single IC together with the pixels. Some of these circuit function comprise, for example, timing generation, signal processing, analog-to-digital conversion and interface. CMOS pixels generally comprise photodiodes although other photo-sensing elements may be used. Another advantage over CCD is the lower voltage and low power requirements of CMOS.
3D imaging systems may be classified into three groups according to the technique used to capture the 3D image of the object or scene: triangulation systems, interferometry systems, and time-of-flight (TOF) systems. A description of a triangulation imaging system may be found in U.S. Pat. No. 6,756,606: “Three Dimensional Imaging by Dual Wavelength Triangulation”, and of an interferometry imaging system may be found in U.S. Pat. No. 5,926,277: “Method and Apparatus for Three Dimensional Imaging Using Laser Illumination Interferometry”, both of which are incorporated herein by reference. Triangulation systems and interferometry systems are generally more complex and costlier than TOF systems, making their use in day to day applications more restricted compared to TOF systems.
TOF systems are typically divided into two classes, modulation type and pulse type. Both classes use a laser source, typically located in a camera, which is directed at the object whose image is to be acquired in 3D. The laser source produces a light, or laser beam, which impinges on the object, a portion of which is reflected back to the image sensor located in the camera. In modulation type systems, a phase of the reflected portion is compared to a phase of the transmitted beam to generate a 3D image. Scanning is typically done one line at a time. Pulse type systems generally use a “scanner-less” method, which comprises generating a short laser pulse having a relatively large field of illumination. The laser pulse may be thought of as a wall of light, which hits the object in the field of view (FOV), a portion of which is reflected back towards the sensor. Pulse type systems are generally preferred over the modulation system as image capture is faster and the use of a mechanical device to perform the scanning is not required.
Pulse type systems require substantially fast discrimination capability and/or relatively high detection speed. Fast discrimination capability requires the use of non-CMOS technologies in the sensor, which makes the sensor rather costly and generally unsuitable for wide commercial applications. High detection speed requires fast gating of the reflected portion entering the camera. This requires that the pixels comprised in the image sensor be substantially simultaneously triggered. This may be achieved using CMOS technology by substantially eliminating the skew time between the pixels.
More information on 3D imaging systems may be found in “A CMOS 3D Camera with Millimetric Depth Resolution”, Cristiano Niclass, Alexis Rochas, Pierre-Andre Besse, and Edoardo Charbon, Swiss Federal Institute of Technology, Lausanne, Switzerland (http://aqua.epfl.ch/PDF/CICC04.pdf); “8.2: A CMOS Smart Pixel for Active 3D Vision Applications”, Luigi Viarani, David Stoppa, Lorenzo Gonzo, Massimo Gottardi and Andrea Simoni, Integrated Optical Sensors Group, ITC-IRST, (vvww.itc.it/soipublications/pub/46.pdf); and “3D Imaging in the Studio (and Elsewhere . . . )”, G. J. Iddan and G. Yahav, 3DV Systems Ltd, Yokneam, Israel, (www.3dvsystems.com); all of which are incorporated herein by reference. Comparative analyses of the advantages in using CCD and/or CMOS may be found in “CCD versus CMOS—has CCD imaging come to an end?”, Nicolas Blanc, Zurich (www.ipf.uni-stuttgart.de/publications/phowo01Blanc.pdf); and “CCD vs. CMOS: Facts and Fiction”, January 2001 issue, Photonics Spectra, Lauren Publishing Co. Inc., (www.dalsa.com/shared/content/Photonics_CCDvsCMOS_Litwiller.pdf); both of which are incorporated herein by reference.